Such a pump unit is used for example in an automatic transmission of a motor vehicle for supplying hydraulic oil. In particular, it is driven by the internal combustion engine so that the quantity of hydraulic oil delivered is governed by the running speed of the internal combustion engine. The geometric pump volume is usually designed with regard to the idling speed. Since the volume delivered increases with the running speed, in the upper speed range such a pump often delivers too much oil. On the one hand this leads to unnecessary energy consumption and on the other hand it can result in cavitation problems such as damage and noise emission. Furthermore, a pump driven exclusively by the internal combustion engine of the motor vehicle cannot deliver any oil when the internal combustion engine has stopped, so that elements actuated by oil pressure are then at least no longer fully functional. The internal combustion engine can stop, for example, because of the start-stop operating mode in which, as an environmental protection measure, the internal combustion engine is switched off for a short time depending on the operating situation, or when driving with the internal combustion engine switched off (“coasting”), or when driving under electric power.
In order nevertheless to enable the pump unit to be used in such operating situations, electrically driven pumps are used as well. Compared with pumps driven by an internal combustion engine such electric pumps are less efficient. Moreover, they take up additional structural space.
DE 102 14 637 A1 describes a pump unit with a hybrid drive. Besides the connection to the internal combustion engine, the hybrid drive also comprises an electric motor. Both the internal combustion engine drive and the electric motor drive are used to power the pump by way of a planetary gearset. The hybrid drive is connected upstream from the pump itself, and this also increases the structural space required.
DE 103 29 215 A1 describes a similar pump unit for which there is also provided a hybrid drive with a summation transmission in the form of a planetary gearset for coupling a first power-train, driven by an internal combustion engine and a second power-train driven by an electric motor, to the pump shaft. Once again, the hybrid drive is connected upstream from the pump housing in this example embodiment and increases the space occupied.
DE 10 2004 005 430 A1 describes a further pump unit which, besides the internal combustion engine drive, also comprises an additional electric motor. The rotor of the electric motor is integrated in the pump housing. Correspondingly, the pump housing is mounted so that it can rotate. This increases the cost and effort of production, in particular also for sealing within the pump. Moreover, this solution can only be used with pumps having an internal gearwheel.